Such a milk pump has, for example, become known from U.S. Pat. No. 6,749,582. In this milk pump, the support is about tubular, the displacement space (and its walls) being able to be plugged into the upper opening of the tube in from above. As far as the displacement space is merely plugged, the interconnection of these parts is, of course, quite unreliable. In a similar way, U.S. Pat. No. 6,652,484 shows a milk pump where a cylindrical displacement space of a manual pumping unit in the shape of a syringe can be screwed off. Also in this case, there is an alternative for connecting a suction hose instead.
In both cases, pumping is rather troublesome; in the first case due to the mere plug connection and the necessity to move a suction membrane upwards, while a cranked actuation lever has to be moved in lateral direction (which leads to friction losses and causes fatigue of the hand); in the second case, because actuating a syringe in two opposite directions requires the use of both hands for pumping, while the milk receptacle has to be held too. Hose connections, however, should be avoided because of the possibility of an entanglement, because of the difficulty to keep it in hygienic conditions, and also because of the tendency of the resilient material of such hoses to become brittle, crumbling or flawy.
Now, a glance should be thrown to the fact, why at all such a releasable connection between the walls of the displacement space and the support is usually made. One reason is the possibility of cleaning separately all pump elements, or in other words to meet higher hygienic standards. Another reason, however, is in both above cases, that one should provide a possibility to use selectively (at least) two types of pumps driven by physical strength. It has been mentioned above that a hose connection is not favorable; however, a screwing connection has also its perils: First removing from a mold during production, e.g. by injection molding, is easy, particularly because the threaded portion, due to the necessary seal, has to be relative long. This leads to higher costs. But the necessary long screwing in practical use is awkward and may, if the two threaded parts are engaged in a wrong manner, lead easily to stripping the thread, which would make the thing unusable. Moreover, screwing does not the use of a curved displacement space for a piston, as is known from WO 01/34226, where the displacement space is integrated into a cap-like support, thus impeding separate cleaning or any exchange of the pumping system, while involving restrictions in the choice of material which should meet both requirements, i.e. that of sufficient strength and that of low friction for the piston, or for a membrane or any other suction member.
A combination of a releasably attachable displacement space and of a support (which, in principle, may be formed in any way desired) permits, however, in an advantageous manner that different types of milk pumps may be brought onto the market, which have all the same support which, in this way, may be produced in greater quantities at reduced costs. Also in this respect, a screw connection would be disadvantageous, because it requires more expenditure of labour when assembling the parts. On the other hand, there existed good reasons why no other connections have been suggested up to now, i.e. because the pumping motion exerts a certain longitudinal force in the direction of movement to the displacement space, thus necessitating a connection as stable as possible. However, the connection should not be so that, when cleaning the parts, the user is hardly able to detach the parts from one another.